Generally, in an electrophotographic type color image forming apparatus, an original picture image is optically or electrically separated in the primary colors. A latent image corresponding to each color of yellow, magenta or cyanogen is formed on a surface of a photoreceptor. The latent image is developed by a toner which corresponds to each color. The developing method of each color is substantially the same as a method for a conventional monochromatic image forming apparatus. However, there are several types for superimposing a toner image of each color on a medium which is to transfer the color image.
A first conventional color image forming apparatus, which is, for example, shown in a Publication Gazette of Unexamined Japanese Patent Application Hei 6-11943, or Hei 3-214173, is comprised of a charging device which is disposed in the vicinity of the outer periphery of a photoreceptor for charging a surface of the photoreceptor, an exposing device for exposing the surface of the photoreceptor selectively, and a plurality of developing devices respectively corresponding to yellow, magenta, cyanogen and black. A paper sheet, to which the color image is to be transferred, is electrostatically attracted on a surface of a transfer drum made of dielectrics. Toner images corresponding to respective colors are superimposed on the surface of the paper sheet by the developing devices.
Operations of the above-mentioned first conventional color forming apparatus are described. Initially, a color document is read by an image scanner, and image information data corresponding to yellow, magenta, cyanogen and black are obtained. These image information data are temporarily stored in a memory. Next, a latent image based on, for example, yellow image data is formed on the surface of the photoreceptor by the exposing device. The latent image on the surface of the photoreceptor is developed by the developing device corresponding to yellow, and a toner image of yellow is formed on the surface of the photoreceptor. In synchronism with the image forming operation on the photoreceptor, a paper sheet is supplied on the transfer drum. The toner image of yellow is electrostatically transferred on the surface of the paper sheet on the transfer drum by a corona discharger. Similarly, toner images of magenta, cyanogen and black are serially formed, and superimposed on the toner image of yellow on the surface of the paper sheet on the transfer drum. The electrostatic attraction force is released by another corona discharger for removing the electric charge. The paper sheet holding the toner images of four colors are peeled off from the transfer drum, the toner images on the paper sheet are fixed by a fixing process, and the paper sheet is ejected from the color image forming apparatus.
In a second conventional color image forming apparatus shown, for example, in a Publication Gazette of Unexamined Japanese Patent Applications Hei 5-61316, Hei 5-181395, Hei 5-119642, or Hei 5-72910, a toner image of each color, which is formed on a surface of a photoreceptor, is transferred to and held on a surface of an intermediate transfer belt in order to prevent trouble due to the paper supply. The toner images are superimposed on the intermediate transfer belt, and transferred to a surface of a paper sheet together. In the second conventional color image forming apparatus, a configuration of a charging device, an exposing device and developing devices which are disposed in the vicinity of an outer periphery of the photoreceptor is substantially the same as the first conventional color image forming apparatus shown in the Publication Gazette of Unexamined Japanese Patent Application Hei 6-11943. The intermediate transfer belt, however, spans a plurality of pulleys. One of the pulleys is driven to contact and separate the surface of the photoreceptor with the intermediate transfer belt. The toner images which are formed on the surface of the photoreceptor are serially transferred to and superimposed on the surface of the intermediate transfer belt in an order of, for example, yellow, magenta, cyanogen and black. When the toner images of four colors are superimposed on the intermediate transfer belt, a paper sheet is supplied in a manner so that the top of the paper sheet and the top of the toner images on the intermediate transfer belt coincide with each other. After that, a transfer roller, which is made of a conductive material and a predetermined voltage is applied, is contacted on the back of the paper sheet for pushing the surface of the paper sheet on the intermediate transfer belt. Thereby, the toner images are transferred to the paper sheet from the intermediate transfer belt. The conductive transfer roller is placed at a position distant from the intermediate transfer belt, while the toner images are superimposed on the intermediate transfer belt. The transfer roller contacts the intermediate transfer belt via the paper sheet only when the toner images on the intermediate transfer belt are transferred to the paper sheet. The paper sheet holding the toner images of four colors passes a fixing process and is exhausted to the outside of the color image forming apparatus.
In the above-mentioned first conventional color forming apparatus, however, the paper sheet is wound on the transfer drum for forming the toner images of the primary three colors and black. Thus, the apparatus includes the processes for electrostatically attracting the paper sheet on a transfer drum and for peeling off the paper sheet from the transfer drum by removing the electrostatic force by the corona dischargers. During the processes, a lot of ozone which is harmful is generated by the corona dischargers. Furthermore, a paper path of the paper sheet becomes complex and the complexity of the paper path can cause a paper jam. In particular, recovery from the paper jam becomes complex.
On the other hand, in the second conventional color image forming apparatus, the toner images are transferred from the photoreceptor to the intermediate transfer belt, and the superimposed toner images are transferred from the intermediate transfer belt to the paper sheet again. Thus, the paper path becomes more simple than that of the first conventional color image forming apparatus. Furthermore, since the electrostatic force by the corona discharger is not used, not as much harmful ozone is generated. However, a mechanism, which separates the transfer roller from the intermediate transfer belt while the toner images formed on the photoreceptor are superimposed on the intermediate transfer belt, is necessary. Thus, the driving mechanisms of the apparatus becomes complex.